A power supply is a device or system that supplies electrical or other types of energy to an output load or group of loads. The term power supply can refer to a main power distribution system and other primary or secondary sources of energy. A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. While a linear regulator uses a transistor biased in its active region to specify an output voltage, a SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage.
SMPS is currently the dominant form of voltage conversion device because of its high power conversion efficiency, small size and weight, and low cost. SMPS takes input power from a source, such as a battery or wall socket, and converts the input power into short pulses according to the demand for power from the circuits coupled to the SMPS output.
MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are commonly used in SMPS. MOSFETs are commonly manufactured separately, as discrete transistors. Each MOSFET is then connected to other integrated circuits that are part of the SMPS. Using discrete devices in this manner increases cost and size of the overall SMPS.
High performing MOSFETs are significant to the conversion efficiency of SMPS because MOSFETs are some of the most power dissipating components in the SMPS. Also, the maximum possible switching frequency of the MOSFETs dictates the size, cost, and power losses in the inductors and capacitors included in the SMPS output filter circuits. Under normal SMPS operation, MOSFETs are turned on and off rapidly, so for efficient operation the MOSFETs should have low values of both resistance and gate capacitance.
A MOSFET has a gate, a drain, and a source terminal, as well as a fourth terminal called the body, base, bulk, or substrate. The substrate simply refers to the bulk of the semiconductor in which the gate, source, and drain lie. The fourth terminal functions to bias the transistor into operation. The gate terminal regulates electron flow through a channel region in the substrate, either enabling or blocking electron flow through the channel Electrons flow through the channel from the source terminal towards the drain terminal when influenced by an applied voltage.
The channel of a MOSFET is doped to produce either an N-type semiconductor or a P-type semiconductor. The drain and source may be doped of opposite type to the channel, in the case of enhancement mode MOSFETs, or doped of similar type to the channel as in depletion mode MOSFETs. The MOSFET utilizes an insulator, such as silicon dioxide, between the gate and the substrate. This insulator is commonly referred to as the gate oxide. As such, the gate terminal is separated from the channel in the substrate by the gate oxide.
When a voltage is applied between the gate and source terminals, the electric field generated penetrates through the gate oxide and creates a so-called “inversion layer”, or channel, at the semiconductor-insulator interface. The inversion channel is of the same type, P-type or N-type, as the source and drain, so as to provide a channel through which current can pass. Varying the voltage between the gate and substrate modulates the conductivity of this layer, which functions to control the current flow between drain and source.
A power MOSFET is a specific type of MOSFET widely used as a low-voltage switch, for example less than 200V. A lateral power MOSFET refers to a configuration where both the drain and the source are positioned lateral of each other, such as both at the top surface of the substrate. This is in contrast to a vertical power MOSFET where the drain and source are stacked vertically relative to each other, such as the source at the top surface of the substrate and the drain at the bottom surface.
One limiting factor in how fast the power MOSFET can be switched on and off is the amount of gate charge needed to turn the transistor on and off. The gate charge refers to the number of electrons that are moved into and out of the gate to turn the transistor on and off, respectively. The larger the needed gate charge, the more time to switch the transistor on and off. There is an advantage to quickly switching the power transistor in a switch-mode power supply. The higher the frequency, the smaller the size of the discrete components used in the gate drive circuit of the SMPS. Smaller components are less expensive than larger components.
FIG. 1 illustrates a cut-out side view of an exemplary configuration of a conventional lateral power MOSFET. In this exemplary configuration, the substrate 10 is doped to form a P-type region, or well, 12 and a N-type region, or well, 14. The P-type well 12 includes a double diffused source 16 having a merged contact 24 between a P+ region 20 and a N+ region 22. The contact 24 shorts the P+ region 20 and the N+ region 22 together. The contact 24 functions as a source contact of the power transistor, and the source is shorted to the body of the substrate, which is P-type in this exemplary configuration. A source contact terminal 42 is coupled to the contact 24, and therefore to the source 16. The substrate 10 is also doped to form a N+ region 18 within the N-type region 14. The N+ region 18 functions as the drain of the power transistor. A drain contact terminal 40 is coupled to the drain 18. A trench 26 is formed in a top surface of the substrate 10. The trench 26 is filled with field oxide. The trench 26 can be formed using Shallow Trench Isolation (STI) and in this case the field oxide filled trench is referred to as a shallow trench isolation (STI) region.
A gate oxide 28 is formed on the top surface of the substrate 10. A polysilicon gate 30 is formed over the gate oxide 28. As shown in FIG. 1, the gate oxide layer 28 between the polysilicon gate 30 and the substrate 10 is a thin oxide layer. The polysilicon gate 30 extends over the STI region to support high drain-to-gate voltage.
There are three main regions in the substrate 10 relative to the operation of the power transistor: a channel region, a transition region, and a drift region. The channel region is formed underneath the polysilicon gate 30 and in the P-type region 12 of the substrate 10. In other words, the channel region is formed where the polysilicon gate 30 overlaps the P-type region 12.
The drift region is the portion of the N-type region 14 underneath the trench 26, or the STI region. The drift region is where most of the drain-to-gate voltage is dropped in the transistor off state. The STI region is necessary to achieve a high drain-to-gate voltage. If the polysilicon gate 30 were to instead terminate over the thin gate oxide, this would result in too high a voltage across the gate oxide and the power transistor would not function. As such, the STI region and the polysilicon gate extension over the STI region are necessary to drop the high gate-to-drain voltage.
The transition region is the portion of the N-type region 14 underneath the gate oxide 28 and the polysilicon gate 30. The transition region provides a current flow path from the channel region to the drift region when the power transistor is turned on. The transition region is also referred to as the accumulation region or the neck region. In many applications, the transition region accounts for the largest single component of on-resistance in a low-voltage power MOSFET. The length of the transition region is an important design consideration, where the length refers to the horizontal direction in FIG. 1. If the length is too short, the on-resistance of the power MOSFET increases, and the device suffers from early quasi-saturation when turned on hard. If the length is too long, the on-resistance saturates, the specific on-resistance increases, and the breakdown voltage drops. The portion of the polysilicon gate 30 positioned over the transition region accounts for a significant portion of the gate capacitance, and therefore the gate charge.